The invention relates to a rotation/linear motion converting mechanism which converts a rotation force to a linear motion, and more particularly to a roller-thread mechanism and to a rotation/linear motion converting mechanism in which the efficiency is high and the distance of linear motion per rotation can be reduced.
Conventionally, a roller-thread mechanism is disclosed in, for example, Japanese Patent Unexamined Publication (Kokai) No. Sho. 59-147151.
As shown in FIG. 1, the roller-thread mechanism is configured by a shaft 20, a nut 21, and rollers 22 and has a mechanism in which the rollers 22 held by a retainer 23 are interposed between the shaft 20 and the nut 22, and, when the shaft 20 is rotated, the nut 21 is linearly moved in the axial direction.
However, such a roller-thread mechanism has problems such as the following.
A) The mechanism is configured so that the nut and the shaft have the same number of threads (sometimes, called stripes) and are identical in direction and the lead of the threads is not smaller than the thread pitch. Therefore, the travel distance of the shaft per rotation of the nut cannot be made shorter than three thread pitches. Specifically, in the case of a roller-thread mechanism, when the rollers and the shaft have the same diameter, the nut has a diameter which is not smaller than the sum of double of the diameter of eath roller and the diameter of the shaft. Furthermore, the number of threads of the nut is an integer. In a usual roller-thread mechanism, therefore, the number of threads of a nut must be three or more. As a result, the travel distance of the shaft per rotation of the nut cannot be made shorter than three thread pitches. PA1 B) In order to shorten the axial travel distance of the roller with respect to the nut, the number of threads of the nut must be greater than that of each roller. However, this configuration increases the lead of the nut. As a result, a rotation power source of a high torque or a reduction gear of a high ratio is required for obtaining a desired axial force. Therefore, such a configuration is disadvantageous to miniaturization of an apparatus. PA1 C) In a roller-thread mechanism in which the nut is not multi-threaded type (that is, the number of threads (stripes) is single), generally, the roller conducts relative axial movement with respect to the nut, and hence a mechanism for retracting the axial position of the rollers for each revolution is required, thereby complicating the configuration.